1. Field of the Invention
The present invention generally relates to a metal/ceramic circuit board and a method for producing the same. More specifically, the invention relates to a metal/ceramic circuit board wherein a metal circuit, such as an aluminum circuit or an aluminum alloy circuit, is formed on a ceramic substrate, and a method for producing the same.
2. Description of the Prior Art
Conventionally, copper plates have been used as metal circuit plates of metal/ceramic circuit boards for power modules. In recent years, in order to realize a higher heat cycle resistance, a metal/ceramic circuit board using an aluminum plate as a metal circuit plate has been proposed and put to practical use.
In order to form an aluminum circuit on such an aluminum/ceramic circuit board, there is known a method for forming an aluminum circuit by etching an aluminum plate, which is bonded to a ceramic substrate, with an etchant of a mixed solution consisting of 30 to 40 wt % of ferric chloride, 5 to 15 wt % of hydrochloric acid and the rest of water (see, e.g., Japanese Patent Laid-Open No. 9-3662).
However, if an aluminum plate is etched with the etchant disclosed in Japanese Patent Laid-Open No. 9-3662, there are some cases where aluminum is too violently dissolved in the etchant to precisely form an aluminum circuit. If the metal circuit plate is made of pure aluminum, it is possible to carry out relatively satisfied etching. However, if the metal circuit plate is made of an aluminum alloy containing Si or the like or if a metal circuit plate of aluminum is bonded to a ceramic substrate via a brazing fitter metal containing Al and Si, there is problem in that the linearity of the peripheral edge of a metal circuit pattern deteriorates, so that the peripheral edge of the metal circuit pattern is corrugated to cause a variation in skirt spreading length on the peripheral edge portion of the metal circuit. Thus, there a problem in that it is not possible to ensure a predetermined distance between metal circuits and a predetermined distance between the surface and reverse of a metal/ceramic circuit board. There is also a problem in that abnormal discharge easily occurs when a module is assembled.